The invention relates to the precise measurement of inductor current, especially for controlling switching in MOS voltage regulator circuits and electric motor controllers.
It is necessary to accurately measure current in order to accomplish control of electric current motors and also to accomplish control of DC-DC converter circuits and voltage regulator circuits. Usually, inductor current measurement for such purposes is accomplished by connection of a low-resistance (e.g., one ohm) resistor connected in series with the inductor conducting the current. U.S. Pat. Nos. 5,731,731 and 5,420,777 disclose this conventional technique. However, providing accurate, low value resistors in an integrated circuit is problematic. Use of the above mentioned series resistor both increases power dissipation of current measurement circuitry and reduces the efficiency of a regulator that includes the current measurement circuitry.
A prior circuit, the MAX887 DC-DC converter marketed by Maxim Integrated Circuits, Inc., measures current through an inductor by connecting a low-resistance resistor between the source electrodes of a relatively large P-channel output transistor and a relatively small P-channel sense transistor having their gate electrodes connected together and also having their drain electrodes connected together, wherein most of the inductor current flows through the output transistor. This technique cannot be used effectively to measure small inductor currents of less than about 10 percent of the maximum inductor current when the output transistor is operating in its so-called xe2x80x9ctriode modexe2x80x9d. This is because the output voltage measured across the resistor for such low inductor currents is a very nonlinear function of the inductor current. Furthermore, the current in the sense transistor is heavily dependent both on MOS or CMOS processing parameters and on the temperature of the integrated circuit chip. Consequently, the inductor current measurement is not as accurate as desirable.
U.S. Pat. No. 5,483,182 discloses circuitry for both measurement of an inductor current and for current limiting of a xe2x80x9clow-sidexe2x80x9d MOS transistor switch in a DC-DC converter. The circuitry includes a reference current source, a comparator, and a sense transistor. The circuit has the shortcoming that it can be used only for current limitation and only for a low-side switch. The circuit would not work properly for, or in conjunction with, the high-side switch of a DC-DC converter, and cannot be used in conjunction with operation of a low-side switch in a synchronous rectifier mode.
Other known current-sensing circuitry, shown in U.S. Pat. No. 5,081,379, has the shortcoming that it is not capable of detecting current if the low-side MOS transistor switch is operated in a synchronous rectifier mode; this prevents the circuit from being useful in measurement of an inductor current connected in an ordinary half bridge connection.
Thus, there is a need for an improved MOS switching regulator circuit, and specifically for an efficient, substantially process-independent, substantially temperature-independent circuit which can be used for precisely measuring inductor current in an MOS switching regulator circuit, motor controller circuit, or the like.
Accordingly, it is an object of the invention to provide an improved MOS switching regulator circuit.
It is another object of the invention to provide an improved circuit for measuring inductor currents so as to make it practical to provide a MOS switching regulator circuit with improved speed and improved stability.
It is another object of the invention to provide an improved circuit for measuring inductor current over a wide range of values.
It is another object of the invention to provide an improved circuit for measuring inductor current over a wide range of values, wherein the measurement is substantially independent of chip temperature and chip processing parameters.
It is another object of the invention to provide an improved circuit for measuring inductor currents so as to make it practical to provide a MOS switching motor control circuit.
It is another object of the invention to provide an improved low power circuit for measuring inductor currents so as to make it practical to provide a MOS switching voltage regulator circuit or a MOS switching motor control circuit.
Briefly described, and in accordance with one embodiment thereof, the invention provides an inductor current measurement circuit (25) accurately measures current supplied to an inductor (17) by a first transistor (2) and/or a second transistor (3). The first transistor (2) has a drain coupled to an input voltage conductor (8), a gate coupled to a switching signal (4), and a source coupled to a the first terminal of an inductor (17). A first sense transistor (7) has a gate and a drain connected to the gate and train, respectively, of the first transistor (2), and a source coupled to a first input of a first amplifier (13) and a drain of a third transistor (12) having a source coupled to a second supply voltage conductor (9), and a gate connected to the gate of a fourth transistor (15). The source of the fourth transistor (15) is connected to the second supply voltage conductive (9) and its drain is connected to an output terminal (18) and a second terminal coupled to the second supply voltage conductor (9). The first transistor (2) and second transistor (3) are included in a DC-DC converter. A second sense transistor (10) has a gate connected to the gate of the second transistor (3) and a drain connected to the drain of the second transistor (3) and to the first terminal of the inductor (17). The source all the second sense transistor (10) is coupled to the output terminal (18). The first and second transistors are operated so that one is on while the other is off, such that the current through the output terminal represents the current through the inductor regardless of which of the first and second transistors supplies the inductor current.